1. Field of the Invention
The present invention relates to an apparatus for initializing a phase-changing optical recording medium, and a phase-changing optical recording medium. More specifically, the present invention relates to an apparatus for initializing a phase-changing optical recording medium wherein a laser beam is irradiated to a recording layer, in an amorphous state, of an optical recording medium to be initialized, which is a pre-stage member of the phase-changing optical recording medium, for changing the amorphous state of the recording layer into a crystalline state with heat of the laser beam, and also relates to a phase-changing optical recording medium.
2. Description of the Related Art
One example of erasable and rewritable optical recording mediums is a phase-changing optical recording medium. A general schematic construction of the phase-changing optical recording medium will be described with reference to FIGS. 7 and 8 which are schematic sectional views of different types of phase-changing optical recording mediums.
As shown in FIG. 7, in a phase-changing optical recording medium of the type that a laser beam emitted from a semiconductor laser enters the medium from the side of a transparent base plate 7, a land 7a and a groove 7b are formed on the principal surface of the transparent base plate 7 in advance. On the transparent base plate 7, a first dielectric layer 8, a recording layer 9 made of a phase-changing material, a second dielectric layer 10, a reflecting layer 11, and a protective film 12 are formed successively in the order named. Also, as shown in FIG. 8, in a phase-changing optical recording medium of the type that a laser beam emitted from a semiconductor laser enters the medium from the side of a transparent cover layer 13, a land 7a and a groove 7b are formed on the principal surface of a transparent base plate 7 in advance. On the transparent base plate 7, a reflecting layer 11, a second dielectric layer 10, a recording layer 9 made of a phase-changing material, a first dielectric layer 8, and the transparent cover layer 13 formed of a resin or film, for example, are formed successively in the order named. The thickness of the transparent base plate 7 or the transparent cover layer 13 is dependent on the NA (Numerical Aperture) of a condensing lens. Correlation between them is such that the thickness decreases as the NA of the condensing lens increases.
In the above structure, the recording layer 9, the first and second dielectric layers 8, 10, and the reflecting layer 11 are generally formed by film forming steps using, e.g., sputtering or vapor deposition. The recording layer 9 is in an amorphous state after the film forming steps, and is then initialized from the amorphous state into a crystalline state. When information is recorded on the phase-changing optical recording medium, a recording mark portion is changed into the amorphous state with the intensity of the laser beam irradiated, while a non-recording portion remains in the crystalline state. In other words, before the user employs a phase-changing optical recording medium, an area of the medium which takes part in recording and reproduction of an information signal is entirely held in the crystalline state.
In the above initializing step, there are two typical methods shown in FIGS. 9 and 10, which conceptually illustrate the initializing process, for changing the recording layer 9 from the amorphous state into the crystalline state. An initializing apparatus is employed to carry out any of the two typical methods. The method shown in FIG. 9 is called a fusion crystallizing process. With this process, the crystalline state is produced by raising the temperature of the recording layer 9 to a level higher than the melting point of the phase-changing material for change from the amorphous state into a molten state, and then slowly cooling it. The method shown in FIG. 10 is called a solid-phase crystallizing process. With this process, the crystalline state is produced by raising the temperature of the recording layer 9 in the amorphous state to a level higher than the crystallizing temperature of the phase-changing material, holding the raised temperature during a period necessary for crystal growth, and then slowly cooling it. In other words, regardless of which one of the methods is employed, the initializing apparatus requires a means for raising the temperature of the recording layer 9 in the amorphous state, for example, a heat source such as a laser beam source. When a laser beam source is employed, the focus of a laser beam is formed by a condensing lens on the recording layer 9 made of the phase-changing material so that the focused area is subject to a large energy density.
FIG. 11 is a schematic view for explaining a process in which the recording layer 9 is changed from the amorphous state into the crystalline state by irradiating a laser beam, which is emitted from a laser beam source and focused through a condensing lens, to the recording layer 9, in the amorphous state, of an optical recording medium to be initialized which is a pre-stage member of the phase-changing optical recording medium.
As shown in FIG. 11, a spot of the laser beam emitted from the laser beam source and focused through the condensing lens is formed on the recording layer 9, in the amorphous state, of the optical recording medium to be initialized which rotates at a predetermined rotational speed. The temperature of an area, in which the beam spot is formed, is raised beyond the crystallizing temperature of the phase-changing material, and is then moved away from the beam spot with the rotation of the initialized optical recording medium for crystallization under slow cooling. Through such a process, the phase-changing optical recording medium is completed in which an area of the medium taking part in recording and reproduction of an information signal is entirely held in the crystalline state.
For bringing the entire area of the medium taking part in recording and reproduction of an information signal into the crystalline state, the energy density of the laser beam, which is emitted from the laser beam source toward the recording layer 9 made of the phase-changing material, must be held constant. Usually, the optical recording medium to be initialized involves an inherent slight warp attributable to flatness of the transparent base plate 7. Therefore, when the optical recording medium to be initialized is, e.g., a disk-shaped medium, there occurs a plane runout on the order of xc2x1500 xcexcm during the rotation. Assuming that the numerical aperture of the condensing lens is NA and the wavelength of the laser beam emitted from the laser beam source is xcex, the focal depth d of the beam spot focused by the condensing lens is expressed by d=xc2x1xcex/2NA2. Given xcex=680 nm and NA=0.45, for example, d=xc2x11.68 xcexcm is resulted. This value is much smaller than the plane runout of xc2x1500 xcexcm that occurs during the rotation of the disk-shaped medium.
For that reason, a focus servo means utilizing the astigmatism process, the Foucault process or the critical angle process, for example, is indispensable in the initializing apparatus for the optical recording medium to be initialized, for the purpose of controlling the beam spot focused through the condensing lens to be always held within the focal depth with respect to the recording layer 9 of the initialized optical recording medium.
However, the initializing apparatus including the focus serve means has two problems as follows.
The first problem is concerned with the condensing lens.
The transparent base plate 7 or the transparent cover layer 13 has a different thickness depending on the type of the optical recording medium to be initialized. To form a normal beam spot without suffering the effect of spherical aberration, therefore, the initializing apparatus must include a dedicated condensing lens in match with the particular thickness of the transparent base plate 7 or the transparent cover layer 13. In other words, it is difficult to initialize various types of optical recording mediums by the initializing apparatus provided with one type of condensing lens.
The second problem is concerned with the focus servo means.
Before coming into the focus servo process, a focus search is carried out in which the condensing lens is moved in the focusing direction and the focus of the condensing lens is locked on to the recording layer 9 of the optical recording medium to be initialized. In the focus search, the surface of the recording layer 9 is generally determined by detecting a peak of the reflection intensity. More specifically, the optical recording medium to be initialized, which has a plurality of films formed thereon one above another, shows peaks of the reflection intensity at two points; i.e., the layered film portion and the surface of the transparent base plate 7 or the transparent cover layer 13. FIGS. 12 and 13 show waveforms of a reflection intensity signal and a focus error signal with the initialized optical recording medium of the type shown in FIG. 7 on condition that the thickness of the transparent base plate 7 is 0.6 mm; FIG. 12 shows the case in which the reflectivity of the recording layer 9 is large and FIG. 13 shows the case in which the reflectivity of the recording layer 9 is small. Also, FIGS. 14 and 15 show waveforms of a reflection intensity signal and a focus error signal with the initialized optical recording medium of the type shown in FIG. 8 on condition that the thickness of the transparent cover layer 13 is 0.1 mm; FIG. 14 shows the case in which the reflectivity of the transparent cover layer 13 is large and FIG. 14 shows the case in which the reflectivity of the transparent cover layer 13 is small.
As is apparent from FIGS. 12 to 15, peaks of the reflection intensity are observed at the respective surfaces of the recording layer 9 and the transparent base plate 7 or the transparent cover layer 13. It is therefore required to separate the peaks and lock the focus of the condensing lens on the surface of the recording layer 9. As shown in FIGS. 14 and 15, however, a reduction in thickness of the transparent cover layer 13 increases difficulties in separating the peaks and hence in locking the focus of the condensing lens on the surface of the recording layer 9. Particularly, as shown in FIGS. 14 and 15, when the reflectivity of the recording layer 9 is smaller than that of the transparent base plate 7 or the transparent cover layer 13, the peak of the intensity of reflection from the recording layer 9 is buried in the peak of the intensity of reflection from the transparent base plate 7 or the transparent cover layer 13. This further increases difficulties in locking the focus of the condensing lens on the surface of the recording layer 9.
An object of the present invention is to provide an apparatus for initializing an optical recording medium to be initialized with stability and ease without employing any focus servo means, and a phase-changing optical recording medium initialized by the initializing apparatus to have a recording layer in a uniform crystalline state over an entire area of the medium which takes part in recording and reproduction of an information signal.
To achieve the above object, according to a first aspect of the present invention, in an apparatus for initializing a phase-changing optical recording medium wherein a laser beam emitted from a laser beam source is irradiated to a recording layer, in an amorphous state, formed in an optical recording medium to be initialized, which is a pre-stage member of the phase-changing optical recording medium, for changing the recording layer from the amorphous state into a crystalline state, the laser beam source, e.g., a YAG laser, includes a high-speed shutter for exciting the laser beam source while the laser beam source is held in a state of not starting oscillation, and emits the laser beam within a very short time on the order of several tens nanoseconds (ns) to several hundreds nanoseconds (ns) with its peak power increased more than 10000 times that produced in a continuous oscillation mode.
According to a second aspect of the present invention, in an apparatus for initializing a phase-changing optical recording medium wherein a laser beam emitted from a laser beam source is irradiated to a recording layer, in an amorphous state, formed in an optical recording medium to be initialized, which is a pre-stage member of the phase-changing optical recording medium, with an optical axis of the laser beam set obliquely relative to the recording layer, for changing the recording layer from the amorphous state into a crystalline state, the laser beam source, e.g., a YAG laser, includes a high-speed shutter for exciting the laser beam source while the laser beam source is held in a state of not starting oscillation, and emits the laser beam within a very short time on the order of several tens nanoseconds (ns) to several hundreds nanoseconds (ns) with its peak power increased more than 10000 times that produced in a continuous oscillation mode.
Also, the present invention provides a phase-changing optical recording medium produced using the apparatus for initializing a phase-changing optical recording medium according to the above first or second aspect.
In the apparatus for initializing a phase-changing optical recording medium according to each of the first and second aspects of the present invention, the laser beam source incident upon the recording layer in an amorphous state is preferably a substantially parallel beam. However, the incident laser beam may be a gently converging or diverging beam that has an energy density enough to change the recording layer from the amorphous state into the crystalline state.
With the above features, the present invention operates as follows.
By employing the laser beam source, including the high-speed shutter, in the apparatus for initializing a phase-changing optical recording medium according to the first aspect of the present invention, the peak power more than 10000 times that produced conventionally can be concentrated in a short time (but enough to raise the temperature of the recording layer for changing from the amorphous state into the crystalline state). As a result, any focus servo means for increasing the power density per unit is no longer required, and the initializing apparatus of the present invention is adaptable for initialization of all types of phase-changing optical recording mediums being different in thickness of a transparent base plate or a transparent cover layer.
Also, by employing the laser beam source, including the high-speed shutter, in the apparatus for initializing a phase-changing optical recording medium according to the second aspect of the present invention, the peak power more than 10000 times that produced conventionally can be concentrated in a short time (but enough to raise the temperature of the recording layer for changing from the amorphous state into the crystalline state). Further, when the laser beam emitted from the laser beam source is irradiated to the recording layer, in an amorphous state, formed in the optical recording medium to be initialized with the optical axis of the laser beam set obliquely relative to the recording layer, the effect of multiple interference occurred between a transparent base plate or a transparent cover layer and the recording layer can be suppressed. As a result, the recording layer having small variations in reflectivity in the crystalline state can be obtained.